The promoters of Ultimate Fighting have damaged their chances of legalizing the sport in New York with a knockout p.r. blunder — having convicted sex offender Michael Boxley participate in an Albany reception aimed at winning votes from state lawmakers, sources told The Post.

Even supporters of the Ultimate Fighting Championship and mixed martial arts were stunned to see Assembly Speaker Sheldon Silver’s disgraced former aide at the Jan. 27 pro-legalization campaign event.

Boxley was convicted in 2003 of sexually abusing a legislative staffer but has re-emerged as a lobbyist. The firm that employs him, Brown & Weinraub, represents UFC-Zuffa Entertainment, the main promoter of MMA bouts.

Opponents say MMA is too violent and showcases misogynistic behavior.

“It couldn’t get any worse from a p.r. standpoint,” said one attendee at the UFC reception. “You don’t want someone convicted of a sex crime at your event. Who’s appearing next, Vito Lopez?”

Lopez, a former Brooklyn assemblyman, resigned last year after probers substantiated claims he sexually harassed female staffers.

UFC confirmed Boxley attended the reception but said he was there only as guest, not a paid advocate.

“Michael Boxley does not lobby for UFC. Michael has never lobbied for UFC,” said spokesman Steven Greenberg, noting he did not participate in the UFC’s press conference and lobbying in the capital the day after.

As for the reception, he said, “there were quite a number of people from Brown & Weinraub who are not lobbyists there. Brown & Weinraub had secretaries and lawyers there. They are a law firm first and foremost.”

While Brown & Weinraub is UFC’s lobbying firm, Boxley is not listed as a lobbyist for the client.

In June 2003, Albany police removed Boxley, then Silver’s chief legal counsel, from the Capitol in cuffs after a female staffer accused him of rape. Boxley quit the Assembly after pleading guilty to misdemeanor sexual misconduct.

The plea required him to register as a sex offender and serve six years’ probation. His law license was suspended. In accepting the deal, he admitted having sex with a 22-year-old legislative staffer without her consent.

A 24-year-old Assembly staffer had filed a sex-abuse complaint against Boxley in 2001, but a probe found no conclusive evidence, and he kept his job.